The effect of HPMC on 3D printing mortar

1.1   Influence of HPMC on the printability of 3D printing mortars

1.1.1   The effect of HPMC on the extrudability of 3D printing mortars

The blank group M-H0 without HPMC and the test groups with HPMC content of 0.05%, 0.10%, 0.20%, and 0.30% were allowed to stand for different periods of time, and then the fluidity was tested. It can be seen that the incorporation of HPMC It will significantly reduce the fluidity of the mortar; when the content of HPMC is gradually increased from 0% to 0.30%, the initial fluidity of the mortar decreases from 243 mm to 206, 191, 167, and 160 mm, respectively. HPMC is a high molecular polymer. They can be entangled with each other to form a network structure, and the cohesion of the cement slurry can be increased by encapsulating components such as Ca(OH) 2. Macroscopically, the cohesiveness of the mortar is improved. With the extension of the standing time, the degree of hydration of the mortar increases. increased, the fluidity lost over time. The fluidity of the blank group M-H0 without HPMC decreased rapidly. In the experimental group with 0.05%, 0.10%, 0.20% and 0.30% HPMC, the degree of decrease in fluidity decreased with time, and the fluidity of mortar after standing for 60 min was 180, 177, 164, and 155 mm, respectively. The fluidity is 87.3%, 92.7%, 98.2%, 96.8%. The incorporation of HPMC can significantly improve the retention ability of mortar fluidity, which is due to the combination of HPMC and water molecules; on the other hand, HPMC can form a similar film It has a network structure and wraps the cement, which effectively reduces the volatilization of water in the mortar and has a certain water retention performance. It is worth noting that when the content of HPMC is 0.20%, the retention ability of the mortar fluidity reaches the highest level.

The fluidity of the 3D printing mortar mixed with different amounts of HPMC is 160~206 mm. Due to the different printer parameters, the recommended ranges of fluidity obtained by different researchers are different, such as 150~190 mm, 160~170 mm. From Figure 3, it can be intuitively seen It can be seen that the fluidity of the 3D printing mortar mixed with HPMC is mostly within the recommended range, especially when the HPMC content is 0.20%, the fluidity of the mortar within 60 minutes is within the recommended range, which satisfies the appropriate fluidity and stackability. Therefore, although the fluidity of the mortar with a suitable amount of HPMC is reduced, which leads to a decrease in extrudability, it still has good extrudability, which is within the recommended range.

1.1.2   The effect of HPMC on the stackability of 3D printing mortars

In the case of not using a template, the size of the shape retention rate under self-weight depends on the yield stress of the material, which is related to the internal cohesion between the slurry and the aggregate. The shape retention of 3D printing mortars with different HPMC contents is given. The rate of change with standing time. After adding HPMC, the shape retention rate of mortar is improved, especially at the initial stage and standing for 20 min. However, with the extension of standing time, the improvement effect of HPMC on the shape retention rate of mortar gradually weakened, which was mainly due to The retention rate increases significantly. After standing for 60 min, only 0.20% and 0.30% HPMC can improve the shape retention rate of mortar.

The penetration resistance test results of the 3D printing mortar with different HPMC contents are shown in Figure 5. It can be seen from Figure 5 that the penetration resistance generally increases with the extension of the standing time, which is mainly due to the flow of the slurry during the cement hydration process. It gradually evolved into a rigid solid; in the first 80 min, the incorporation of HPMC increased the penetration resistance, and with the increase of the content of HPMC, the penetration resistance increased. The greater the penetration resistance, the deformation of the material due to the applied load The greater the resistance of HPMC is, which indicates that HPMC can improve the early stackability of 3D printing mortar. Since the hydroxyl and ether bonds on the polymer chain of HPMC are easily combined with water through hydrogen bonds, resulting in the gradual reduction of free water and the connection between particles increase, the friction force increases, so the early penetration resistance becomes larger. After standing for 80 minutes, due to the hydration of cement, the penetration resistance of the blank group without HPMC increased rapidly, while the penetration resistance of the test group with HPMC increased The rate did not change significantly until about 160 min of standing. According to Chen et al., this is mainly because HPMC forms a protective film around the cement particles, which prolongs the setting time; Pourchez et al. conjectured that this is mainly due to fiber Simple ether degradation products (such as carboxylates) or methoxyl groups can delay cement hydration by retarding the formation of Ca(OH)2. It is worth noting that, in order to prevent the development of penetration resistance from being affected by the evaporation of water on the surface of the specimen, This experiment was carried out under the same temperature and humidity conditions. On the whole, HPMC can effectively improve the stackability of the 3D printing mortar at the initial stage, delay the coagulation, and prolong the printable time of the 3D printing mortar.

3D printing mortar entity (length 200 mm × width 20 mm × layer thickness 8 mm): The blank group without HPMC was severely deformed, collapsed and had bleeding problems when printing the seventh layer; The M-H0.20 group mortar has good stackability. After printing 13 layers, the top edge width is 16.58 mm, the bottom edge width is 19.65 mm, and the top-to-bottom ratio (the ratio of the top edge width to the bottom edge width) is 0.84. Dimensional deviation is small. Therefore, it has been verified by printing that the incorporation of HPMC can significantly improve the printability of mortar. Mortar fluidity has good extrudability and stackability at 160~170 mm; shape retention rate is less than 70 % is seriously deformed and cannot meet the printing requirements.

1.2   Influence of HPMC on rheological properties of 3D printing mortars

The apparent viscosity of pure pulp under different HPMC content is given: with the increase of shear rate, the apparent viscosity of pure pulp decreases, and the phenomenon of shear thinning is under high HPMC content. It is more obvious. The HPMC molecular chain is disordered and shows higher viscosity at low shear rate; but at high shear rate, HPMC molecules move in parallel and orderly along the shear direction, making the molecules easier to slide, so the table The apparent viscosity of the slurry is relatively low. When the shear rate is greater than 5.0 s-1, the apparent viscosity of P-H0 in the blank group is basically stable within 5 Pa s; while the apparent viscosity of the slurry increases after HPMC is added, and it is mixed with HPMC. The addition of HPMC increases the internal friction between the cement particles, which increases the apparent viscosity of the paste, and the macroscopic performance is that the extrudability of the 3D printing mortar decreases.

The relationship between the shear stress and shear rate of the pure slurry in the rheological test was recorded, and the Bingham model was used to fit the results. The results are shown in Figure 8 and Table 3. When the content of HPMC was 0.30%, the shear rate during the test was greater than 32.5 When the viscosity of the slurry exceeds the range of the instrument at s-1, the corresponding data points cannot be collected. Generally, the area enclosed by the rising and falling curves in the stable stage (10.0~50.0 s-1) is used to characterize the thixotropy of the slurry [21, 33]. Thixotropy refers to the property that the slurry has great fluidity under the action of external force shearing, and can return to its original state after the shearing action is cancelled. Appropriate thixotropy is very important to the printability of the mortar . It can be seen from Figure 8 that the thixotropic area of the blank group without HPMC was only 116.55 Pa/s; after adding 0.10% of HPMC, the thixotropic area of the net paste increased significantly to 1 800.38 Pa/s; With the increase of , the thixotropic area of the paste decreased, but it was still 10 times higher than that of the blank group. From the perspective of thixotropy, the incorporation of HPMC greatly improved the printability of the mortar.

In order for the mortar to maintain its shape after extrusion and to withstand the load of the subsequent extruded layer, the mortar needs to have a higher yield stress. It can be seen from Table 3 that the yield stress τ0 of the net slurry is significantly improved after HPMC is added, and it is similar to HPMC. The content of HPMC is positively correlated; when the content of HPMC is 0.10%, 0.20%, and 0.30%, the yield stress of the net paste increases to 8.6, 23.7, and 31.8 times that of the blank group, respectively; the plastic viscosity μ also increases with the increase of the content of HPMC. 3D Printing requires that the plastic viscosity of the mortar should not be too small, otherwise the deformation after extrusion will be large; at the same time, a suitable plastic viscosity should be maintained to ensure the consistency of material extrusion. In summary, from the point of view of rheology, HPMC’s Incorporation has a positive effect on the improvement of stackability of 3D printing mortar. After incorporating HPMC, the pure paste still conforms to the Bingham rheological model, and the goodness of fit R2 is not lower than 0.99.

1.3   The effect of HPMC on the mechanical properties of 3D printing mortar

28 d compressive strength and flexural strength of 3D printing mortar. With the increase of HPMC content, the 28 d compressive and flexural strength of 3D printing mortar decreased; when the content of HPMC reached 0.30%, the 28 d compressive strength and The flexural strengths are 30.3 and 7.3 MPa, respectively. Studies have shown that HPMC has a certain air-entraining effect, and if its content is too high, the internal porosity of the mortar will increase significantly; The diffusion resistance increases and it is difficult to discharge all. Therefore, the increase of porosity may be the reason for the decrease of the strength of 3D printing mortar caused by HPMC.

The unique lamination molding process of 3D printing leads to the existence of weak areas in structure and mechanical properties between adjacent layers, and the bonding strength between layers has a great influence on the overall strength of the printed component. For 3D printing mortar specimens mixed with 0.20% HPMC M-H0.20 was cut, and the interlayer bond strength was tested by the interlayer splitting method. The interlayer bond strength of the three parts was higher than 1.3 MPa; and when the number of layers was low, the interlayer bond strength was slightly higher. The reason may be that, on the one hand, the gravity of the upper layer makes the lower layers more densely bonded; on the other hand, the surface of the mortar may have more moisture when printing the lower layer, while the surface moisture of the mortar is reduced due to evaporation and hydration when printing the upper layer, so The bonding between the bottom layers is stronger.

1.4   Effect of HPMC on the Micromorphology of 3D Printing Mortar

The SEM images of the M-H0 and M-H0.20 specimens at 3 d age show that the surface pores of the M-H0.20 specimens are significantly increased after adding 0.20% HPMC, and the pore size is larger than that of the blank group. This On the one hand, it is because HPMC has an air-entraining effect, which introduces uniform and fine pores; on the other hand, it may be that the addition of HPMC increases the viscosity of the slurry, thereby increasing the discharge resistance of the air inside the slurry. The increase may be the main reason for the decrease in the mechanical properties of the mortar. To sum up, in order to ensure the strength of the 3D printing mortar, the content of HPMC should not be too large (≤ 0.20%).

In conclusion

(1) Hydroxypropyl methylcellulose HPMC improves the printability of the mortar. With the increase of the content of HPMC, the extrudability of the mortar decreases but still has good extrudability, the stackability is improved, and the printable The time is prolonged. It has been verified by printing that the deformation of the bottom layer of the mortar is reduced after adding HPMC, and the top-bottom ratio is 0.84 when the HPMC content is 0.20%.

(2) HPMC improves the rheological properties of 3D printing mortar. With the increase of HPMC content, the apparent viscosity, yield stress and plastic viscosity of the slurry increase; thixotropy first increases and then decreases, and the printability is obtained. Improvement. From the perspective of rheology, adding HPMC can also improve the printability of the mortar. After adding HPMC, the slurry still conforms to the Bingham rheological model, and the goodness of fit R2≥0.99.

(3) After adding HPMC, the microstructure and pores of the material increase. It is recommended that the content of HPMC should not exceed 0.20%, otherwise it will have a great impact on the mechanical properties of the mortar. The bonding strength between different layers of the 3D printing mortar is slightly different, and the number of layers When it is lower, the bond strength between mortar layers is higher.